This invention relates generally to a method and apparatus for speed control of steam turbines. More specifically, the invention relates to a method for overcoming performance degradation of a worn or defective pilot-valve assembly (a component of the control system) by employing one or more additional, digital controllers; thus improving the overall accuracy of the turbine speed-control system.
To govern the speed and power of a steam turbine, a valve (or more commonly, a set of valves) must be adjusted to vary the flow of steam through the turbine. Typically, such valves are regulated with a hydraulic steam-valve actuator which, in turn, is activated by way of a pilot valve modulated by an electromechanical actuator that receives its signal from a speed-control system.
Present-day speed control systems for steam turbines include a proportional-integral-differential (PID) controller that utilizes signals representing rotational speed. This speed controller then transmits an actuator-position set point to another PID controller that monitors steam-valve actuator position and whose output activates (indirectly) the steam-valve actuator to render its position equal to the actuator set point. In reality, the steam-valve actuator controller""s output is employed as a set point for an electromechanical actuator which modulates a pilot valve: hydraulic fluid is directed through the pilot valve to-and-from the steam-valve actuator to change its position. Pilot valves can, however, suffer performance degradation due to manufacturing defects, wear, and other ills, thereby impairing system performance. Consequently, a method of control that compensates for faulty pilot valves is needed.
A purpose of this invention is to provide a method for controlling the rate of steam flow through a steam turbine by monitoring the position of a pilot valve along with the dynamics of a steam valve, and using this information to compensate for the action of a faulty pilot-valve assembly that does not perform to standard.
To accomplish this purpose, control elements are added to the standard control system used to govern turbine speed. In particular, one or two additional PID controllers are included. One of these units is dedicated to maintaining the position of the pilot valve at a set point obtained from a PID steam-valve actuator position controller. Therefore, the controller for pilot-valve position is cascaded with the controller for steam-valve position.
A second controller is dedicated to steam-valve actuator velocity. For that reason, a calculation function is required, which takes the first time-derivative of the steam-valve position signal. And the set point for this controller is proportional to the difference (error) between the steam-valve position set point and its actual position.
The resulting signal, inputted to the pilot-valve""s electromechanical actuator, is proportional to a linear combination of the outputs from the two additional PID controllers.